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Friday, June 12, 2009

H1N1 Pandemic: Stay Calm, and Stay Home if Ill

The latest WHO figures give a total of 29699 confirmed cases af AH1N1 world wide, with 145 deaths.

This gives a case mortality rate of 0.5% - that is, you have a 0.5% chance of dying if you get it.

How does this compare with ordinary seasonal flu?Here we are given a clinical attack rate (i.e. the proportion of the population that catches flu) of 5-15% for seasonal flu. Let's call that 10%, which would give 30,400,000 cases out of a total USA population of 304,000,000.

Annual death rates due to seasonal flu are around 30,000 in the USA, giving a case mortality rate of 0.1%.

Therefore case mortality rates are five times higher for pandemic flu.

Because less people have antibodies to the new strain, the clinical attack rate is higher with pandemic flu, 25-50% as against 5-15% for seasonal flu - 3-5 times. Let's take that as 4 times higher. That would give 5*4= 20 times the number of deaths that seasonal flu causes - 120,000 in the USA. I make that 0.04% of the US population. Individually each a tragedy (especially if pandemic hits younger people harder) but statistically the figures are not highly significant.

I stress that these are very crude calculations indeed, but they give us a ball park figure of what to expect. The actual numbers could be several times higher or several times lower.

The mortality rate for H1N1 may have been held down by the high level of vigilance and high use of antivirals in these early cases. Stocks are not infinite, so later on people may have to face the virus without support from drugs. In fact, their effect is to reduce the severity and duration of symptoms by a not very large margin. I would be very interested to see them compared with high dose Vitamin C and Zinc, which supports the immune system. There is some evidence for its efficacy. (I know, Dr Goldacre, not a lot, but this is because nobody gets a patent for vitamin C and Zinc).

Key messages are to reduce the case attack rate by emphasising the Stay Home If Ill message. Politically, employers are stupidly going to resist this message, on the grounds that absenteeism will increase. In fact, absenteeism in the longer run will be worse if ill people come in to work and spread it around. Government should help by abolishing the duty of GPs to issue medical certificates, since this drives infectious people out into the community to get their sick notes.

There should also be air quality monitors in civil airliners that could be tested and signal whether the aircraft was carrying H1N1. The passengers could be contacted, and advised to regard themselves as infectious for a week.